The risk of liver diseases due to alcohol and toxin abuse and hepatitis viruses in US Veterans is increasingly high and is one of the most common reasons for hospitalization and mortality. Hepatic fibrotic disease represents one of the largest groups of disorders for which there is no effective therapy and thus denotes a major unmet medical need. Often the only option for patients with liver fibrosis is organ transplantation. Chronic liver diseases include cholangiopathies that target cholangiocytes such as Primary Sclerosing Cholangitis (PSC) which is characterized by biliary proliferation, inflammation and progressive fibrosis. Unrestrained cholangiocyte proliferation can develop into cancer of the bile ducts (i.e., cholangiocarcinoma, CCA) and patients with PSC are more susceptible to development of CCA. Further, cholangiocytes display a senescent phenotype during PSC which may contribute to inflammation and further influence hepatic fibrosis by activating hepatic stellate cells (HSCs). It has been shown that damaged cholangiocytes secrete senescence-associated secretory phenotypes (SASP). Mast cells (MCs) are important in mediating numerous pathologies including liver diseases, but are found at very low numbers in normal, homeostatic livers. Infiltrating hepatic MCs are found near damaged intrahepatic bile ducts and activated HSCs. In unpublished data, we have found that damaged, senescent cholangiocytes induce MC migration during non-alcoholic fatty liver disease. Further, senescent cholangiocytes secrete factors like stem cell factor (SCF) and interleukins that are known to be chemoattractants for MCs, inducing migration. Following migration and activation, MCs release mediators including large amounts of histamine that stimulates cholangiocyte proliferation and fibrosis. The rationale for our proposal is built upon previously published data from our lab and others showing that MC infiltration increases in PSC and CCA patients along with rodent models of liver damage, and MC infiltration positively correlates with increased fibrosis. Additionally, normal wild-type mice (typically very few hepatic MCs) injected with cultured MCs display increased biliary damage, inflammation and hepatic fibrosis, all of which are key features of PSC. Using a model of PSC (Mdr2-/- mice) we generated a double knockout mouse (DKO) by breeding Mdr2-/- with mice lacking histidine decarboxylase (HDC-/-). DKO mice (few to no MCs) have decreased biliary damage, inflammation and hepatic fibrosis. Further, upon reintroduction of MCs into DKO mice, we find a striking increase in damage and fibrosis that mimics Mdr2-/- mice, which was reversed when MCs lacking TGF-?1 signaling were used demonstrating a key role for MCs in PSC. Finally, inhibition of MC-derived histamine decreases biliary damage and hepatic fibrosis in models of cholestatic liver injury, PSC and CCA suggesting that modulation of MCs mediators may prove therapeutic. We propose the working hypothesis that senescent cholangiocytes induce MC migration during PSC and modulation of MC-derived TGF-?1 or FXR regulates ductular reaction and hepatic fibrosis via biliary senescence. We propose the following specific aims (SAs): (SA1) Senescent cholangiocytes recruit MCs to the liver by secretion of specific SASPs during PSC; (SA2) MCs promote liver and biliary damage, inflammation and hepatic fibrosis resembling PSC in normal mice or mice lacking MCs; and (SA3) MCs exacerbate biliary damage and hepatic fibrosis during PSC via cellular crosstalk between histamine, TGF-?1 and FXR.